Collaborative Research: Imaging Stress Transients and Fault Zone Processes with Continuous Cross-Well Active Source Seismic Measurements at SAFOD

非技术摘要地震是由断层上突然释放的应力引起的。板块构造描述了长期应变积累的过程，但应力释放的细节--最终导致断层破裂的原因，以及地震(无论是大地震还是小地震)、震动或缓慢滑动的结果--仍然没有被很好地理解。地震开始处深部随时间变化的应力/应变场是控制这些地震/非地震事件的序列和成核的最重要的性质之一。预测的应力变化成功地解释了余震的空间分布，并为大地震事件的聚集提供了一种解释，如附近的1992年兰德斯地震和1999年赫克托矿震，说明了形变瞬变的重要性。然而，压力的测量是出了名的困难，特别是在深度上。大地测量学对地表变形场提供了重要的约束，这可以通过假设的流变学与应力有关。然而，大地测量对应力和应变深度分布的限制是有限的。这些表面约束需要与其他技术相结合，这些技术虽然与应力和应变没有直接关系，但具有更高的深度分辨率。目前最有希望的技术似乎是地震技术。例如，长期以来，地震活动模式一直被用来推断地震事件前后的应力状态。然而，这种方法工作的一个基本要求是，随着时间的推移，应力的变化会产生可以在地表观察到的地震波，这意味着不能以这种方式观察到应力和/或应变的无地震变化。评估这种完全无地震成分的一种方法是通过观察地壳弹性性质的时间变化。事实上，这种与地震相关的变化早就被预测到了，有时还会被观察到，这是由于应力导致充液裂缝的特征和或分布发生了变化。然而，由于小信号水平和可能产生明显变化的其他系统效应的问题，例如震源位置的变化或浅层环境影响，很难确凿地观察到介质中的时间变化。我们正在利用SAFOD(圣安德烈亚斯深部断层观测站)导航孔和主孔进行连续的井间有源地震实验。这项实验的广泛和长期目标是开发一种工具，通过检测孕震深度的地壳速度结构的时间变化，监测与地震和其他依赖应力的地球过程有关的时变应力场，例如无地震滑动和非火山震动。这项技术将是一种“压力计”。这样的监测系统可能是了解地震和非地震事件触发过程的最重要的单一手段。地震应力计背后的基本物理原理已经确立。几十年来的大量实验室研究表明，地震速度明显表现出应力相关性，通常归因于裂缝物理特征的变化(如裂缝密度、裂缝方向)。我们目前的实验是在2005年和2006年在SAFOD现场进行的先前实验的基础上进行的。在两个月的时间里，我们观察到横波通过飞行员和主孔之间岩石所需时间的变化(几微秒)与气压变化(约1千帕卡)之间的负相关。这一结果是对我们实验场地震速度应力敏感性的一次“校准”。我们还观察到旅行时间数据中的两个大漂移，这两个漂移与两次地震--一次3级地震和一次1级地震--重合，这两次地震发生的距离足够近，足以在SAFOD地点产生较大的同震应力变化。这两次漂移分别在3级和1级地震前大约10小时和2小时开始，这表明它们可能与早期实验室研究中观察到的破裂前裂缝性质的变化有关。在目前的实验中，我们正在使用类似的设备配置来收集当地10-15级2到3级地震的样本数据。我们正在使用一种基于互相关的方法和尾波干涉测量技术来成像SAFOD站点附近介质中的系统变化，并使用它们来监测加利福尼亚州帕克菲尔德附近断裂带过程的时间变化。该项目对地震研究和地球范围计划具有重要意义。它将证明在即将到来的地震的震源区附近，在破裂之前是否有可测量的地震速度结构的变化。因此，这项工作可以显著提高我们对地震前物理过程的理解。这将代表着在测量与地震和其他过程有关的应力瞬变方面取得的重大进展，表明可以通过连续的活跃震源钻孔观测来连续监测构造应力。

Non-technical SummaryEarthquakes are caused by the sudden release of stresses along faults. Plate tectonics describes the process of long-term strain accumulation, but the specifics of stress release -- what ultimately leads to fault failure and whether an earthquake (small or large), tremor, or slow slip results -- are still not well understood. The time-varying stress/strain field at the depths where earthquakes begin is one of the most important properties controlling the sequencing and nucleation of these seismic/aseismic events. The importance of deformation transients has been illustrated by the success of predicted stress changes in accounting for the spatial distribution of aftershocks, as well as providing one explanation for the clustering of large seismic events, such as the nearby 1992 Landers and 1999 Hector Mine earthquakes. The measurement of stress, however, is notoriously difficult, particularly at depth. Geodesy provides important constraints on the surface deformation field, which can be related to stress through an assumed rheology. Yet, the constraints on the depth distribution of stress and strain from geodesy are limited. These surface constraints need to be combined with other techniques that, while not as directly related to stress and strain, have superior depth resolution. The most promising techniques at present appear to be seismic. For example, patterns of seismicity have long been used to make inferences about the stress state before and after seismic events. Yet, a basic requirement for this approach to work is that changes in stress over time generate seismic waves that can be observed at the surface, which means that aseismic changes in stress and/or strain cannot be observed in this manner. One way of assessing this fully aseismic component is through observations of temporal changes in the elastic properties of the crust. Indeed, such earthquake-related changes have long been predicted and sometimes observed, due to stress-induced changes in the characteristics and or distribution of fluid-filled cracks. Yet, it has been difficult to conclusively observe temporal variations in the medium, due both to the small signal level, and to the problem of accounting for other systematic effects that may produce apparent changes, such as variations in source location or shallow environmental influences. Technical DescriptionWe are conducting a continuous cross-well active-source seismic experiment utilizing the SAFOD (San Andreas Fault Observatory at Depth) pilot and main holes. The broad, long term goal of this experiment is to develop a tool to monitor the time-varying stress field associated with earthquakes and other stress-dependent earth processes, such as aseismic slips and non-volcanic tremors through the detection of temporal changes in the crustal velocity structure at seismogenic depths. This technique would be a type of "stress meter". Such a monitoring system would perhaps be the single most important means of understanding the triggering processes of seismic and aseismic events. The fundamental physics behind a seismic stress meter is well established. Numerous laboratory studies over several decades have shown that seismic velocities clearly exhibit stress dependence, usually attributed to changes in the physical characteristics of cracks (e.g. crack density, crack orientation). Our current experiment is built on a previous experiment we conducted at the SAFOD site in 2005 and 2006. Over a two-month period, we observed a negative correlation between changes in the time required for a shear wave to travel through the rock between the pilot and main hole (a few microseconds) and variations in barometric pressure (about 1 kilopascal). This result is a "calibration" of the stress sensitivity of seismic velocity at our experiment site. We also observed two large excursions in the travel-time data that are coincident with two earthquakes, a magnitude 3 and a magnitude 1 earthquake, that occurred sufficiently close to produce large coseismic stress changes at the SAFOD site. The two excursions started approximately 10 and 2 hours before the magnitude 3 and 1 earthquakes, respectively, suggesting that they may be related to pre-rupture changes in crack properties, as observed in the early laboratory studies. In the current experiment, we are using a similar equipment configuration to collect data that sample 10-15 magnitude 2 to 3 local earthquakes. We are using a cross-correlation based method and the coda wave interferometry technique to image systematic changes in medium near the SAFOD site, and use them to monitor temporal changes in fault zone processes near Parkfield, CA. This project has important implications for the study of earthquakes and for the EarthScope program. It will demonstrate whether there are measurable changes in seismic velocity structure near the source region of an impending earthquake immediately preceding the rupture. As such, this work could lead to significant improvement in our understanding on physical processes prior to earthquakes. It will represent significant progress toward measuring stress transients associated with earthquakes and other processes, demonstrating that tectonic stress could be continuously monitored with continuous active source borehole observations.